1. Field of the Invention
The present invention generally relates to memory technology. In particular, the present invention relates to the fabrication of metal-doped chalcogenides.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and the like.
U.S. Pat. No. 6,084,796 to Kozicki, et al., entitled “Programmable metallization cell structure and method of making same,” discloses another type of nonvolatile memory device known as a programmable conductor memory cell or a programmable metallization cell (PMC). U.S. Pat. No. 6,084,796 is herein incorporated by reference in its entirety. Such memory cells can be integrated into a memory device, which has been referred to as a programmable conductor random access memory (PCRAM). A chalcogenide glass element is doped with metal, preferably silver (Ag). Application of an electric field with a first polarity causes a conductive pathway to grow along the sidewalls or in the sidewalls of the glass element, whereas an electric field of the opposite polarity dissolves the conductive pathway back into the glass element. If the conductive pathway extends between electrodes at opposite ends of the glass element, the resulting short or relatively low resistance can represent a logic state, e.g., a “1” state for the memory cell, whereas the unshorted, relatively high resistance state can represent another logic state, e.g., a “0” state. Additional applications for a programmable metallization cell include use as a variable programmable resistance and a variable programmable capacitance.
One conventional technique for producing the programmable conductor memory cell applies silver (Ag) photodoping to a chalcogenide glass such as germanium selenide (Ge3Se7). The silver (Ag) photodoping process deposits silver (Ag) over germanium selenide (Ge3Se7) and exposes the underlying substrate assembly to a relatively intense source of ultraviolet (UV) radiation for an extended period of time, such as 15 minutes. Disadvantageously, the photodoping process is relatively time-consuming and can slow semiconductor fabrication rates. The photodoping process can decrease the overall process rate especially when it is repetitively applied, such as in the fabrication of a multiple layer stack. Further disadvantageously, the extended exposure to intense UV radiation can induce the glass to convert from an amorphous material to a crystallized material, which thereby results in reduced yields.
Another disadvantage to producing memory cells with silver (Ag) photodoping of glasses is that relatively precise control of the amount of silver (Ag) that is photodiffused into the glass is necessary. A sufficient amount of silver (Ag) must be incorporated into the glass backbone and yet, the glass must not crystallize. If too much silver (Ag) is photodiffused into the glass, the glass crystallizes. If too little silver (Ag) were to be photodiffused into the glass, the memory cell would not switch properly.
Another disadvantage to the photodoping process is that the ultraviolet light is attenuated by the silver film as the ultraviolet light penetrates through the silver film. Such attenuation varies exponentially with the thickness of the film. In one example, with 300 nanometers (nm) wavelength ultraviolet radiation, the intensity of the ultraviolet radiation decreases to only about 10% of its initial intensity after penetrating through 650 angstroms (Å) of silver film. This attenuation renders photodoping to be impractical with relatively thick films, and requires relatively precise control of the thicknesses of the silver (Ag) and chalcogenide glass films. In order to form a thick film with a UV photodoping process, the UV photodoping process is disadvantageously applied repetitively to relatively thin films of silver (Ag). In addition, the varying attenuation of the ultraviolet light continues as the silver (Ag) dopes the chalcogenide glass. Further disadvantageously, this attenuation in intensity of the ultraviolet light as the ultraviolet light penetrates material results in a nonuniform depth profile of the doped silver (Ag) in the chalcogenide glass.